Process for the preparation of precipitated silica

ABSTRACT

Crystalline precipitated silica is produced by hydrolyzing silicon tetrachloride in the presence of fluoride ions at a temperature of up to 60° C.

The invention relates to a process for the preparation of precipitatedsilica by hydrolysis of a silicon compound in an acid medium.

It is known that silicas are prepared on an industrial scale either bythe wet process or pyrogenically, the preparation of precipitatedsilicas by the wet process predominating. Waterglass (aqueous alkalimetal silicate solution) is used as the starting material for theproduction of precipitated silicas, and the precipitation of the silicais carried out with the aid of mineral acids, in particular sulphuricacid (see Ullmanns Encyklopadie der technischen Chemie (Ullmann'sEncyclopaedia of Industrial Chemistry), 4th edition, 1982, Volume 21,page 462 et seq.). The main disadvantage of this process is thedifficulty of obtaining a highly pure silica which, in particular, isfree from alkali.

It is furthermore known that the preparation of alkali-free silica byhydrolysis of silicon halides is possible; however, direct hydrolysis ofsilicon tetrachloride always leads to the formation of a silica gel (seeGmelins Handbuch der anorganischen Chemie (Gmelin's Handbook ofInorganic Chemistry), 8th edition, 1959, Volume 15, part B, page 463).This gel has the disadvantage that it cannot be filtered without furthermeasures.

The object of the invention is to develop a process for the preparationof precipitated silica which can be carried out in a simple manner andleads to a product which is as pure as possible.

The invention relates to a process for the preparation of precipitatedsilica, characterized in that silicon tetrachloride is hydrolyzed at atemperature of not more than 60° C. in the presence of a compound whichdonates fluoride ions, and the crystalline hydrolysis product isseparated off and dried.

Commercially available silicon tetrachloride with a degree of purity ofat least 99 percent by weight is used as the starting material for theprocess according to the invention. The use of freshly distilled silicontetrachloride which has a degree of purity of at least 99.9 percent byweight is particularly advantageous.

The silicon tetrachloride is hydrolyzed in the presence of a compoundwhich donates fluoride ions. Suitable compounds are, in particular,silicon tetrafluoride, hexafluorosilicic acid and, preferably, hydrogenfluoride. The hydrogen fluoride is advantageously used in the form ofdilute aqueous hydrofluoric acid, the HF content preferably being 10 to50 percent by weight. Introduction of the silicon tetrachloride into anaqueous medium in which the fluoride ions are present in an amount of0.3 to 3, preferably 0.5 to 1.5, percent by weight is advantageous.

The hydrolysis of the silicon tetrachloride is carried out at atemperature of not more than 60° C. A temperature in the range from 20°to 50° C. is particularly advisable.

When the hydrolysis has ended, the crystalline product formed isseparated off from the reaction mixture, preferably by centrifugation orfiltration. If appropriate, the hydrolysis product is then washed withwater.

The crystalline silica obtained by hydrolysis is finally dried. Dryingis preferably carried out in two stages, and in particular initially ata temperature of 100° to 130° C., preferably 110 to 120° C., and then ata temperature of 600° to 900° C., preferably 650° to 850° C.

The precipitated silica prepared according to the invention isdistinguished by a high purity. It is largely free from alkali metalsalts and has an extremely low content of transition metals; inparticular, the iron content is not more than 5 ppm. The silica issuitable, above all, as the base material for luminous substances andfor glass optical fibres and insulating mixtures.

The following examples serve to illustrate the invention in more detail.Percentages in each case relate to the weight.

EXAMPLE 1

100 1 of water were taken in a 150 litre polyethylene container, and 2 1of hydrofluoric acid (50 percent strength) were added. 30 kg of freshlydistilled silicon tetrachloride (purity 99.9 percent) were stirred intothis mixture in the course of 8 hours the temperature of the mixturebeing kept at 40° C. Thereafter, the mixture was stirred for a further 3hours, the temperature being kept at 40° C. The silica suspension thusobtained was centrifuged and the silica separated off was washed on afilter with 50 l of water. The silica was then first dried at atemperature of 120° C. for 24 hours and then calcined at a temperatureof 800° C. for 2 hours. 10.4 kg (95 percent of theory) of silica wereobtained in the form of a finely crystalline powder. The iron content ofthe silica was 1.3 ppm.

EXAMPLE 2

1,200 l of water were taken in a 2,000 liter polyethylene container and25 1 of hydrofluoric acid (48 per cent strength) were added. 245 kg ofcommercially available silicon tetrachloride were stirred into thismixture in the course of 16 hours, the temperature of the mixture beingkept at 45° C. Thereafter, the mixture was stirred for a further 6hours, the temperature being kept at 35° C. The silica suspension thusobtained was centrifuged and the silica separated off was washed on afilter with 2,000 l of water. The silica was then first dried at atemperature of 110° C. for 24 hours and then calcined at a temperatureof 700° C. for 3 hours. 80 kg (92.5 percent of theory) of silica wereobtained in the form of a finely crystalline powder. The iron content ofthe silica was 1.5 ppm.

What is claimed is:
 1. The process for the preparation of crystallineprecipitated silica which comprises hydrolyzing silicon tetrachloride atat temperature of up to 60° C. in the presence of a fluoride iondonator, separating the crystalline hydrolysis product from the reactionmixture and then drying the crystalline silica.
 2. The process accordingto claim 1 wherein the silicon tetrachloride starting material has adegree of purity of at least 99.9 percent by weight.
 3. The processaccording to claim 1 wherein hydrogen fluoride is the fluoride iondonator.
 4. The process according to claim 1 wherein hydrolysis is in anaqueous medium in which fluoride ions are present in an amount of 0.3 to3 per cent by weight.
 5. The process according to claim 1 whereinhydrolysis is carried out at a temperature of 20° to 50° C.
 6. Theprocess according to claim 1 wherein drying is in two stages, the firststage at a temperature of 100° to 130° C. and the second stage at atemperature of 600° to 900° C.